This has changed 2 years ago after the American elections. They have removed the so-called "link customization" feature - which made it possible sharing individual images from a web application - in an attempt to "fight back newsfeed manipulations". (As they said) They promised to offer some alternative solution but they did not as far as I know. It's good for their business anyway - people don't spend much time on external sites and they will share photos through Facebook.
Thanks for the reply. I solve the problem, but damn, FB is really.... (no comment).
It was working normally yesterday, but today I have been forced into the FB tool to add an icon, the URL of confidentiality and if my site was with a pro usage.